functor_list.sml

signature LIST =
sig
    type t

    val toList : t list

    val head   : t option
    (* structure Tail : LIST *)

    val empty  : bool
end

functor Nil(E : sig type e end) =
struct
    type t = E.e

    val toList = []

    val head = NONE
    structure Tail = struct type e = E.e end

    val empty = true
end

functor Cons(E : sig structure Tail : LIST
                     val x : Tail.t end) =
struct
    type t = E.Tail.t

    val toList = E.x :: E.Tail.toList

    val head = SOME E.x
    structure Tail = E.Tail

    val empty = false
end

structure Xs = Cons(struct val x = 5
                           structure Tail =
                              Cons(struct val x = 19
                                          structure Tail =
                                              Cons(struct val x = 27
                                                          structure Tail =
                                                              Nil(struct
                                                                    type e = int
                                                                  end)
                                                   end)
                                   end)
                    end)